I'm going to do this review a little bit differently today. I'm going to review two books at once. This is not to take away from either of these two books. They will both get their due. However, since they are on a similar theme, I thought they might play off each other well. Since there seems to be a bit of an upswing right now in traditional martial arts, there might be more people looking to start an art. Whenever that happens the first question is always "what art should I study?" But lets assume we got past that point.
Whenever anyone starts a new art, there seems to be such a large body of knowledge to try and absorb. There's new physical movements, there's new social organization, and new philosophies/principles, and all of that is usually in a foreign language (for us in the English speaking world). These books both represent a really good starting point for their respective arts.

Content

Both of these books, in a different format, go through the basics of their martial art. They cover all the starting information that anyone new to the art would have questions on. They both cover the basic history, and philosophy of their art. They cover what you should know over the first couple lessons, including typical behavior/rituals involved in a class, and even something like how to dress yourself in these funny looking pajamas (and skirt for aikido). They then go through some of the most basic techniques, such as those typically taught at the beginning of a karate or aikido curriculum. They both end with a simple dictionary for words usually used in aikido or karate classes, and include a list of common books and websites for further research.

Pros

The thing I like the most about these books is that their information is presented without prejudice for a specific school/style. The author of Karate Basics, Robin Rielly, has a seventh dan in Shotokan Karate, but the information he presents is generalized enough to be applicable to any karate student. When the information presented is specific to Shotokan, Mr. Rielly implicitly states this. And these style specifics are only brought up as examples to provide a baseline. Aikido Basics follows this same pattern.

Cons

The only thing I would comment on negatively about these two books, are that they represent themselves very well in their titles. They cover the basics. If you are well read in karate, or aikido, then these books may not hold your interest. But in their defense, they literally put that in their titles.

Conclusion

Overall I think these are really well done books. It is always a difficult place for an expert to write a "basics" book. The questions is always going to be "how much information before I overload them?" These two books find that balance point very nicely. There are three groups of people that I think will find these books the most useful. The first is those who are just starting a martial art, in this case either karate or aikido, and want to take an active part in their own martial education. The second group would be those who have studied a martial art, say karate, for a little while, and are starting to branch out in their martial arts curiosity. They want to find out a little about what those crazy aikidoka are up to, and why they roll around like that. The third group, and I'm a part of this group, are going to be instructors that want to have an easy reference material available to new students. The price is honestly, pretty cheap ($10) for the amount of material covered. I'm going to buy multiple copies, and give Aikido Basics out as part of the "starting package" for new students. Here's your gi, your belt, and your book.
For those reasons I'm going to give both of these books four out of five ninja stars. If you are in one of the groups of peoples I mentioned above, then this book would be extremely useful. If you're not, then they might not be as helpful.